Handle AddDesktopButton's visibility in different launcher states

Introduce [ADD_DESK_BUTTON] in LauncherState and change the button's
visibility across different launcher states. E.g., the button
shouldn't be visible in the quick switch.

Bug: 389209338
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Flag: com.android.window.flags.enable_multiple_desktops_backend
Test: 1. Swipe up from home, the button shown util in overview.
      2. Swipe up from the fullscreen app, the button is invisible
         util release the finger.
      3. The button is invisible in quick switch.
      4. All the above tested with NexusLauncher and Nova launcher,
         which is a 3rd launcher.
      5. Tested with gesture and 3-finger swipe on the touchpad.
      6. Both the ClearAllButton and AddDesktopButton are not
         visibile when in select mode.
Change-Id: I54e11d4d03401be68b5b67a6f591edd737c9ea30